It is known, for instance from commonly owned U.S. Pat. No. 5,042,235 to shrink wrap palletized goods by first placing a thermoplastic lower foil atop a pallet with edges of the foil projecting laterally past the pallet and then stacking goods on the pallet atop the foil on the pallet within the edges of the foil. The lower edge of a downwardly open heat-shrinkable foil hood is held as the hood is fitted downward over the stack on the pallet until the lower edge is generally below the stack. This lower edge is then gripped below the stack and the foil hood is shrunk from top to bottom around the stack while continuing to grip the lower edge below the stack. Finally the foil hood is welded to the outer edge of the lower foil. Thus when the grippers are carried on the same vertically displaceable frame as the heater/blowers it is possible for the grippers to release and move upward before the top-to-bottom shrinking of the hood. During such shrinking the clamps will prevent the hood or sack from riding up too much.
The apparatus for doing this has as described in U.S. Pat. No. 5,018,339 a transport device defining a support surface for displacing the palletized goods in a transport direction and including an upstream conveyor, a station conveyor spaced downstream by an upstream gap from the upstream conveyor, and a downstream conveyor spaced downstream by a downstream gap from the station conveyor. A heat ring has upstream and downstream portions and side portions extending in the direction therebetween. This ring is displaceable between a lower position with the upstream and downstream portions in the gaps below the surface and the side portions flanking the station conveyor and an upper position above the surface. An actuator displaces the ring between its upper and lower positions. Thus the ring need merely be moved upward once the pallet is positioned on the station conveyor within it to preshrink the wrap and drive out the air in it, then is moved down once to its starting position to fully shrink the wrap in place.
While these systems are relatively effective, they take some time shrinking the hood around a large stack of palletized goods. In fact this shrink-wrapping operation can constitute a substantial bottleneck in production.